Notes for Plan-Assumption:

Issue: This definition is in some literal sense true, but we may
not want to force the existence of all these assertions with instantiated
variables. Also, would need to link the Actor having the Assumption with
respect to planning, not just at all. THis is why it cannot be an Iff-Def

Slots on relation Plan-Assumption:

A Relationship between an Plan and a Assumption whereby the Plan is
based on the Assumption (i.e. used during Planning) and may be invalid if
the Assumption fails to hold
An Assumption in a Plan-Assumption relationship is also the Assumption in a
Planning-Assumption Relationship for the Planning Activity that produced the
Plan